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200A-4 [message #21494] Sun, 21 September 2014 15:36 Go to next message
norwegian is currently offline  norwegian
Messages: 10
Registered: October 2007
Location: Norway
Junior Member
I have an old 200 A-4 with fuses in the front.
I have problems with hiss. Stevem suggested to replace the first gain stage transsitors after the input jack on both channels.
Does anybody know the specs for these transistors and where to buy them?

Re: 200A-4 [message #21495 is a reply to message #21494] Sun, 21 September 2014 19:07 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4728
Registered: June 2004
Location: NY
Senior Member
The industry standard to this day for that transistor is 2N3904.

Note that if you go to this sites technical section to look at the 200A schematic by model you will bring up the earlier type 200 head and not a 200A!

If you look up the schematics by the board number on your circui you will get the right schematic!

Both preamp board use the 2N3904 transistor.

If your 200A has no effects it uses 8 of these transistors alone, 4 on each board.
Also on each board is a 2N4249 transistor which is a PNP type transistor.

Before you order any replacement transistors I would first go to the rear wall of amp where the 702. Board is mounted.
On it's right side as you face it you will find two blue wires right next to each other.
I would unsolder these one at a time and see if the hiss goes away, if it does that is a good sign as it means the hiss issue that you have is coming from one or both of the preamp boards or the effects boards.
If the hiss is still there than one or more of the other transistors on that 702 board are making noise.
Let use know how this checks first!

[Updated on: Sun, 21 September 2014 20:39]

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Re: 200A-4 [message #21496 is a reply to message #21495] Sun, 21 September 2014 21:40 Go to previous message
norwegian is currently offline  norwegian
Messages: 10
Registered: October 2007
Location: Norway
Junior Member
Thanks Stevem! I will give it a try!
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